1. Field of the Invention
The present invention relates to the monitoring of vehicular traffic and particularly to the provision of signals commensurate with traffic flow through a region of interest of a multi-lane highway. More specifically, this invention is directed to apparatus for detecting the presence, direction of motion and speed of vehicles passing through a precisely defined space and especially to apparatus for providing traffic condition information which does not require the installation of sensor elements in or on the pavement along which the traffic of interest is moving. Accordingly, the general objects of the invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
The automated communication of information commensurate with traffic conditions in real time to a central location, to thereby enable traffic control management decisions to be made and implemented, is becoming of increasing importance. Changes in average traffic speed may be indicative of a localized problem which warrants action, the activation of signs for example, to alert motorists to a potentially dangerous condition that is being approached and/or to change the posted legal operating conditions. At the present time, sensors which provide signals from which traffic flow information may be derived are mounted in or on the pavement. Such sensors, for permanent installation, are typically in the form of a pair of spaced magnetic loop detectors embedded in the pavement. In the case of temporary installations, such as in construction areas, such sensors may take the form of spaced pneumatic tubes which are secured to the surface of the pavement. These prior art sensors provide a pair of "switch closures", i.e., two electrical pulses spaced by a variable time period, for each vehicle detected.
The use of sensors which are installed in or on the pavement presents a number of significant problems. In the case of the magnetic loop detectors which are embedded in the pavement, installation is expensive and requires the creation of a dangerous condition while the traffic lane in which the sensor "antenna" is being installed is closed to permit such installation. Furthermore, because the operating environment is exceedingly harsh, particularly where the pavement is subjected to large variations in temperature, such loop detectors are prone to breakage. Repair of a broken loop detector presents the same problems as the initial installation. In the case of pneumatic tubes, which are directly subject to impact by vehicles, service life is relatively short due to the harsh operating conditions.
It has been suggested that radiant energy, and particularly conventional Doppler radar, could be used to replace the above-discussed loop antennas and surface installed sensors. However, loop antennas and surface installed sensors have the ability to detect moving traffic in a single lane of travel whereas the prior art devices which employ radiant energy, such as Doppler radar transceivers, do not have the ability to accurately detect motion within a single traffic lane only. A further deficiency of previously available vehicle detectors employing microwave radar results from the fact that such radar devices produce beams which are characterized by side-lobes. Reflection of the side-lobe energy is known to produce "false" vehicle detections. Additionally, the central traffic control station data processing equipment which is presently installed is typically capable of using information in the form of the above-discussed "switch closures" and the output signals provided by present microwave radar detectors is not compatible with such existing data processing equipment.
In addition to the above-mentioned criteria, it is essential that a traffic detector employed in a freeway management system provide only valid information, i.e., the detector must be able to discriminate between vehicular traffic moving in a direction of interest and stray reflections, movement of animals, wrong way traffic, etc. Similarly, in order to be suitable for a freeway management system application, a motion detection and discrimination device and technique must be highly reliable and have only modest power supply requirements.